1. Field of the Invention
The present invention relates to power saving measures in diaphragm driving control of a camera system and a lens apparatus.
2. Description of the Related Art
The presently used digital single lens reflex camera includes a live view function (moving image checking function) which enables a user to check captured images by sequentially displaying image signals read from an image sensor on a display unit such as a liquid crystal display (LCD) mounted on the back of the camera.
In addition, an exposure value during shooting of a still image is determined by metering light of the captured scenes. Further, an exposure value of an image displayed as a live view image on the display unit such as an LCD is controlled by an automatic exposure control function.
Generally, during a live view function operation, a number of diaphragm driving times is reduced as much as possible to prevent a deterioration in focus detection accuracy and to secure quietness, and a diaphragm is controlled with full aperture priority.
However, when shooting a scene in a high luminance state, live view needs to be displayed by maintaining an aperture in a reduced state. That is, the image sensor has to be used as both an imaging sensor and a light metering sensor to realize the live view function in the digital single lens reflex camera. In other words, since the image sensor having a restricted dynamic range is used as the light metering sensor, the aperture needs to be reduced in order to prevent saturation.
Further, when a still image is captured with a camera system which includes no live view function, an aperture needs to be reduced in order to detect a flicker phenomenon occurring due to an illuminance change of a captured scene. In this case, in a diaphragm driving device which drives a diaphragm by a stepping motor, power consumption is increased since a current has to be continuously applied to maintain the aperture reduced state.
Unless the current is continuously applied, the reduced state of the aperture cannot be maintained, which causes a change in an aperture reducing position. In other words, an aperture becomes different from a set aperture, which causes an error in a light metering value and accuracy of exposure control is reduced.
This phenomenon is not only applied to the digital single lens reflex camera equipped with the live view function. Also in a camera system equipped with a diaphragm control device which drives a diaphragm by a stepping motor, the above described phenomenon occurs if the camera is configured to enter a power cut-off state after a diaphragm button is pressed to reduce an aperture.
Japanese Patent Application Laid-Open No. 2002-156681 discusses a camera system which drives the diaphragm by the stepping motor. The camera system discussed there opens a diaphragm again to execute light metering anew if a shooting instruction is received while a diaphragm button is pressed, and carries out shooting while the current is being applied.
In the camera system discussed in Japanese Patent Application Laid-Open No. 2002-156681, upon reception of the shooting instruction while the diaphragm button is pressed, exposure control is carried out cutting off the current to avoid power consumption after an aperture is reduced. The diaphragm is opened again when light metering is executed anew. This operation causes a release time lag since the light metering is executed all over again.
Therefore, the above-described problem cannot be solved by the discussed camera system. The same holds true for the camera system equipped with no live view function.